From MPI-3 11.5.1:

- The MPI call MPI_WIN_FENCE(assert, win) synchronizes RMA calls on win. - The call completes an RMA access epoch if it was preceded by another fence call and the local process issued RMA communication calls on win between these two calls. - The call completes an RMA exposure epoch if it was preceded by another fence call and the local window was the target of RMA accesses between these two calls. - The call starts an RMA access epoch if it is followed by another fence call and by RMA communication calls issued between these two fence calls. - The call starts an exposure epoch if it is followed by another fence call and the local window is the target of RMA accesses between these two fence calls. MPI_Win_fence may both start or complete an RMA epoch but it may also complete-one-and-start-another RMA epoch depending on the context and the assertions. All of the tests below are valid in my opinion. What the assertions below in the first example are saying is that the first call to MPI_Win_fence need not complete any RMA calls, which is true because it is the first sync call. The last MPI_Win_fence asserts the same thing in reverse. The middle one asserts nothing because it is both completing and starting an epoch. I think that MPI_Win_fence is poorly designed but that ship has sailed and I believe that the usage is well-defined in the standard despite the confusing properties of this function. Jeff On Fri, Feb 8, 2013 at 11:36 AM, Nick Radcliffe <[email protected]> wrote:

Hi,

I have a question about MPI RMA, and the ANL regression tests for RMA in particular. The tests mixedsync.c and epochtest.c seem to have contradictory views of fence synchronization.

epochtest.c seems to suggest that access/exposure epochs opened by a call to MPI_Win_fence are not closed until a call to MPI_Win_fence with assert==MPI_MODE_NOSUCCEED. The test looks roughly like this:

MPI_Win_fence(MPI_MODE_NOPRECEDE, win) if (rank == src) MPI_Put MPI_Win_fence(0, win) if (rank == dest) MPI_Put etc... MPI_Win_fence(MPI_MODE_NOSUCCEED, win)

Since there is a call to MPI_Put after the second call to MPI_Win_fence, it would seem that the second call could not have ended the access epoch for dest, or the exposure epoch for src (which is the target of the second Put).

On the other hand, the test mixedsync.c looks roughly like this:

if (rank == src) { MPI_Win_lock(...,win) MPI_Put MPI_Win_unlock(...,win) }

MPI_Win_fence(0, win) if (rank == src) MPI_Put MPI_Win_fence(0, win)

if (rank == src) { MPI_Win_lock(...,win) MPI_Put MPI_Win_unlock(...,win) }

The problem is that it is erroneous to call MPI_Win_lock on a window while that window is exposed due to a call to MPI_Win_fence. If mixedsync.c is not erroneous, then the second call to MPI_Win_fence must end the exposure epoch on win, contradicting what's implied about fence synchronization by epochtest.c.

Sorry for the long post, but if anyone can shed some light on this for me, I would greatly appreciate it.

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-- Jeff Hammond Argonne Leadership Computing Facility University of Chicago Computation Institute [email protected] / (630) 252-5381 http://www.linkedin.com/in/jeffhammond https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond